This invention relates to a composition for positive correction of lithographic printing plates. In particular, the invention relates to a correction fluid applicable by a pen or a brush. The invention also relates to a correction kit containing the composition and methods for adding image areas to a printing plate.
The art of lithographic printing is based upon the immiscibility of oil and water. The oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is applied, the background or non-image area retains the water and repels the ink, while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth, and the like. Typically, the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Lithographic printing plates typically comprise a radiation-sensitive coating applied over the hydrophilic surface of a support. If after exposure to radiation the exposed regions of the coating become soluble or dispersible in a developer and are removed in the developing process, revealing the underlying hydrophilic surface of the support, the plate is called a positive-working printing plate. Conversely, if the exposed regions of the plate become insoluble in the developer, and the unexposed regions are removed by the developing process, the plate is called a negative-working plate. Independent of the specific type of printing plate (positive working or negative working plate imaged by the use of a mask, plates which are directly imageable by a laser, i.e. without using a mask, organic photoconductive plates, etc.), the image area (i.e. the imagewise remaining parts of the coating after the developing of the plate) is ink-receptive or oleophilic and the non-image area or background is water-receptive or hydrophilic.
If the developed plate shows xe2x80x9cerrorsxe2x80x9d in the form of missing image areas, it is time consuming and expensive to prepare a new plate. Therefore, additive correction fluids have been developed that add further image areas and make the printing plate suitable for use. These fluids can be applied to non-image areas of a plate to make corrections or to add signs and marks thereon.
Various methods for the correction of lithographic printing plates have been disclosed. DE-A-3717653 [Pliefke, U.S. Pat. No. 4,834,844] describes a method for the electrolytic deposition of organic compounds on the aluminum oxide layer. In the examples epoxides, acrylics, melanin resins and polymers based on maleic acid are mentioned as suitable organic compounds. DE-B-2418789 [Hamilton, U.S. Pat. No. 4,150,623] describes a method for the deposition of carbon particles on the aluminum oxide layer at high temperatures. However, these methods are cumbersome to use.
In another method, pens are used for additive correction. These pens work by mechanical rubbing on the plate surface. By this, aluminum oxide is removed and the underlying aluminum then functions as ink receptive area.
In yet another method, pens containing a polyvinylphenolic derivative are used. However, these pens suffer from the disadvantage that image areas obtained by them have only a low press life. Furthermore, the solvent used in the pens may attack the coating of the plate making correction within an image area extremely difficult.
Previously known correction fluids suffer from the disadvantage that they do not show good adhesion to all known substrates, i.e. substrates having different interlayers. A particular correction fluid can only be successfully used with a particular type of plate so the printer must have a different type of correction fluid for each type of printing plate.
Usually, the developed printing plates are subjected to a preheat or a baking step at a temperature of about 50 to about 300xc2x0 C., preferably about 100xc2x0 C. to about 250xc2x0 C. in order to improve the print run stability. Correction fluids based on polymers different from novolac resins (which are often used in the coating of plates) often do not withstand such temperatures.
For large size corrections, the correction fluid is preferably applied by a brush instead of a pen; however, most of the known fluids cannot be applied by a brush but only by a pen.
Thus a need exists for a correction fluid for lithographic printing plates that is applicable to all kinds of plates independent of the type of interlayer present, shows good adhesion on all types of interlayers, can be baked, and produces image areas having a high print run stability. Furthermore, the correction fluid should dry quickly, have no dissolving effect on the image areas of the plate and should preferably be applicable by pen and brush.
In one embodiment, the invention is an additive correction fluid comprising:
(a) a film forming aliphatic and/or aromatic ester of phosphoric acid; and
(b) an organic solvent or solvent mixture that does not dissolve the image areas of a plate to be corrected.
In another embodiment, the invention is a method for adding image areas to a printing plate. In still another embodiment, the invention is a kit comprising the additive correction fluid.
The first essential component of the additive correction fluid is a film forming aliphatic and/or aromatic ester of phosphoric acid (hereafter also referred to as the xe2x80x9cfilm forming phosphatexe2x80x9d or simply xe2x80x9cphosphatexe2x80x9d).
The phosphate may have further functional groups selected from the group consisting of epoxy groups, hydroxy groups, C1-C10 alkoxy groups (in which the alkyl moiety may be substituted by a C6-C20 aryl group), aryloxy groups (in which the aryl moiety comprises 6-20 carbon atoms), xe2x80x94SO3R (in which R is selected from the group consisting of hydrogen, C1-C10 alkyl and C6-C20 aryl), xe2x80x94Oxe2x80x94SO3R (in which R is as defined above), halogen, xe2x80x94NO2, xe2x80x94Oxe2x80x94PO(OR)2 (in which each R independently is as defined above), xe2x80x94COOR and xe2x80x94CONR2 (in which each R independently is as defined above), carbonyl groups and mixtures thereof.
Suitable phosphates are commercially available from Lubrizol Corporation, Wickliffe, Ohio, USA, and Lubrizol Coating Additives GmbH, Ritterhude, Germany, under the trade names LUBRIZOL(copyright) 2061 (a epoxy-functional phosphate), LUBRIZOL(copyright)2062 (an aliphatic/aromatic phosphate) and LUBRIZOL(copyright) 2063 (a hydroxy/carboxy functional phosphate). These phosphates are available in the form of highly viscous fluids comprising ethyleneglycol monobutyl ether.
The second essential component is an organic solvent, which does not dissolve the image areas of the plate to be corrected. Organic solvent also refers to solvent mixtures, i.e. mixtures of various organic solvents.
In principle, any organic solvent or solvent mixture conventionally used in lithographic printing inks as well as any organic solvent commonly used for cleaning printing plates may be used. Preferably, the solvent is selected from aliphatic and aromatic hydrocarbons, including terpenes and mixtures thereof. In connection with the drying properties of the correction fluid, it is preferred that the organic solvent or solvent mixture has an evaporation number measured according to DIN 53170 of about 1 to about 200, preferably about 1 to about 160, more preferably about 1 to about 50.
Especially preferred solvents are o-, m- or p-xylene and mixtures thereof, white spirit, toluene, and benzene. White spirit, or solvent naphtha, is a petroleum fraction comprising mainly hydrocarbons that boil in the range of about 135 to about 210xc2x0 C. White spirit is available in various boiling ranges, for example about 145 to about 160xc2x0  C., about 150 to about 195xc2x0 C., about 140 to about 170xc2x0 C., about 135 to about 180xc2x0 C., about 180 to about 210xc2x0 C., etc. It is preferred that aromatic hydrocarbons, especially benzene, be removed from the white spirit.
The correction fluid may further contain a dye or pigment for indicating the correction. Any dye or pigment usually used for increasing the image contrast in the radiation sensitive coating for a printing plate can be used provided it is soluble in the solvent of the correction fluid or easily introduced as a pigment in the disperse form. Suitable dyes and/or pigments include, for example, rhodamine dyes, methyl violet, anthraquinone pigments, and phthalocyanine dyes and/or pigments. Especially preferred are Sudan Red (C.I. 26110; 2,3-dimethyl-4-(2-hydroxy-1-naphthylazo)azobenzene), Sudan Blue II (C.l. 61554; 1,4-bis-(butylamino)anthraquinone), Sudan Black (C.I. 26150; [4-(benzene(azo)-naphthyl-1-azo]-2,2-dimethyl-2,3-dihydroperimidine and Solvent Green 3 (C.I. 61565; anthraquinone).
The correction fluid may contain one or more further conventional additives selected from those well known to those skilled in the art, such as, antioxidants, dispersion agents, wetting agents, and antifloating and antiflooding agents.
The amount of the film forming phosphate is preferably about 10 to about 50% by weight based on the total correction fluid. If the correction fluid is intended to be applied with a pen, the amount of the phosphate is preferably about 10 to about 40% by weight based on the total correction fluid, more preferably about 15 to about 30% by weight. If the application is done by a brush (i.e. in the case that a relatively large image area has to be added), the amount of the phosphate is preferably about 30% to about 50% by weight based on the total correction fluid, more preferably about 35 to about 45% by weight.
The amount of the organic solvent or solvent mixture is preferably about 50 to about 90% by weight based on the total correction fluid. For application by pen, the amount is preferably about 60 to about 90% by weight, more preferably about 60 to about 70% by weight. For application by brush, the amount is preferably about 50 to about 70% by weight, more preferably about 50 to about 60% by weight.
The dye and/or pigment is present in an amount of about 0 to about 2% by weight based on the total correction fluid, preferably about 0.05 to about 0.8% by weight, more preferably about 0.1 to about 0.5% by weight.
For brush application it is preferred to add about 2 to 10% by weight of a thickener soluble in the solvent or solvent mixture to increase the viscosity.
Preferred thickeners are AEROSIL(copyright) 380 finely divided silica and AEROSIL(copyright) 200 finely divided silica, which are pyrogenic silica acids available from Degussa, Germany.
For application by a pen, the viscosity of the correction fluid is preferably from about 1.5 to about 5.5 mPaxe2x97xafs (measured with ball viscometer and ball No. 2), more preferably from about 2.5 to about 5.3 mPaxe2x97xafs. For brush application it is preferably about 5 to about 12 mPaxe2x97xafs, more preferably from about 5.3 to 11.4 mPaxe2x97xafs.
The correction fluid can be used for correction of conventional positive working and negative working plates imaged by the use of a mask, plates that are directly imageable by a laser, i.e. without using a mask, organic photoconductive plates, etc. Preferably, the plates have a metal substrate, such as an aluminum substrate, or a polymer substrate, such as polyethylene terephthalate, coated with metal.
The correction fluids show good adhesion on all types of interlayers. They can be applied to printing plates having a polyvinyl phosphonic acid interlayer (PVPA), a phosphate/fluoride interlayer, a PVPA-copolymer interlayer, a silicate interlayer, or an interlayer of a copolymer of vinylphosphonic acid and maleic acid anhydride. Plates having been pretreated with HCI or HNO3 together with phosphate fluoride and plates having a LOMAR(copyright) condensed naphthalene sulfonic acid (Henkel, Dxc3xcisseldorf, Germany) substrate can also be corrected. Furthermore, they have a high print run stability and can be baked at usual temperatures. In addition, the correction fluids do not dissolve the coating of the image areas and dry quickly.
Complete drying of the plate to be corrected is not necessary before the correction fluid is applied. Some residual moisture is not detrimental as to conventional correction fluids. Thus, another advantage of these correction fluids is that they can be applied to a plate already mounted on a press and wetted with fountain solution without the necessity of complete drying of the plate.
Depending on the size of the image area to be added, the correction fluids of the invention are applicable by pen and by brush. The suitable viscosity for each application is obtainable by the selection of the amount of solvent and phosphate and the optional use of a thickener for application by brush where necessary.
The invention also relates to a method for adding image areas to a printing plate. The method comprises applying a correction fluid according to the invention to a printing plate being in need of correction and drying the added image areas.
It is preferred that the corrected plate is subjected to a preheat or a baking step at a temperature in the range of about 50 to about 300xc2x0 C. In a further preferred embodiment, the correction fluid is applied to the plate while the plate is mounted on a press.
The invention also provides a kit comprising
a) a first container, the first container containing a correction fluid, the correction fluid comprising:
30 to 50% by weight of a material selected from the group consisting of film forming aliphatic esters of phosphoric acid, film forming aromatic esters of phosphoric acid, film forming aromatic and aliphatic esters of phosphoric acid, and mixtures thereof; and
50 to 70% by weight of an organic solvent that does not dissolve the image areas;
(b) a second container, the second container containing the same organic solvent as in the first container;
(c) a brush adapted to apply the correction fluid to the plate: and
(d) a refillable pen.
A user of such a kit may apply the correction fluid directly with the brush. A further possibility is that the user may fill the refillable correction pen with the correction fluid. If the viscosity of the correction fluid is too high for application with the pen (i.e. more than about 5 mPaxe2x97xafs measured with a Hxc3x6ppler ball viscometer using ball No. 2), the user can further dilute the correction fluid with organic solvent from the second container and fill the pen with the diluted fluid.
The refillable pen can be any pen known in the art for applying correction fluids, for instance pens having a tank like pens having a valve or pens having a tampon inside. Valve markers comprise a container that is closed with a valve. Upon slight pressure on the correction tip, the valve opens and releases correction fluid, which is then transferred to the plate via the fiber rod (tip). Correction fluid is absorbed by a felt tampon and transferred to the plate via a fiber rod tip (capillary effect). Suitable pens are for instance commercially available from Ratioplast GmbH, Schwabach, Germany.
Suitable application systems with a brush are for instance commercially available from Geka-brush GmbH, Bechhofen-Waizendorf, Germany, and Lxc3x6ffler Kunststoffwerk GmbH, Freyung-Linden, Germany. Any conventionally used application system can be used as far as its components are resistant to the solvent of the correction fluid. For refillable pens, an aluminum case is preferred.
Industrial Applicability
The correction fluid can be used to correct all types of lithographic printing plates, independent of the type of interlayer present. It has good adhesion to a variety of interlayers, can be baked, and produces image areas having a high print run stability. Furthermore, the correction fluid dries quickly, has no dissolving effect on the image areas of the plate, and is applicable by pen and brush.
The advantageous properties of the invention may be observed by reference to the following examples, which illustrate, but do not limit the invention.